1. Field of the Invention
The present invention relates to a composition of matter for use in tribological applications, and more specifically relates to the use of such a composition in mechanical seals, bearings and other sliding or rubbing components which require good durability and wear characteristics.
2. Background Art
Material science has long been used as a basis for choosing materials for components having surfaces which are in, or may come into, sliding contact with each other. For example, in the field of mechanical face seals, a primary ring made of a carbon or carbon like material and a mating ring made of a harder material, such as silicon carbide, tungsten carbide, alumina, stainless steel, etc., are generally known. For a general survey of the materials which have been used and for a description of and the desirable characteristics of those materials for use in the rings of mechanical face seals, see A. O. Lebeck, Principles and Design of Mechanical Face Seals, John Wiley & Son, Inc., New York, N.Y., 1991, pp. 78-94.
The particular emphasis of this invention is directed to the "softer" of the two sliding members, e.g., the "carbon" primary ring of a mechanical face seal. The conventional method of producing material for carbon bodies used for the softer of the two members involves a number of steps. First, carbon fillers are mixed together with a carbonaceous binder. The carbon fillers may be selected with the desired particle size distribution from calcined petroleum coke, metallurgical coke, synthetic graphite, natural graphite, lampblack, carbon black and different chars. A char may be defined as a carbonaceous material which has been carbonized in an inert gas or in an oxygen depleted atmosphere. Also, all raw materials contain certain amounts of impurities. For example, certain types of graphites may contain as much as 20% of such impurities, which are commonly referred to as ash.
The binder can be any organic material which readily decomposes and has a relatively high carbon residue upon heating in an inert atmosphere. Good candidates for binders include coal tar, coal tar pitch, synthetic and natural resins, molasses and sugars. Raw material mixing takes place at either room or elevated temperatures, depending upon the binder characteristics.
After mixing, the carbon aggregate is cooled if needed, then crushed and milled to the desired fineness. Then the carbon bodies are formed in a die having the desired shape using a hydraulic press or are isostatically pressed in rubber molds. The formed "green bodies" are then heated in an inert atmosphere from room temperature, raising the temperature at a rate as low as 1.degree. C. per hour at the critical gas evolution stage. The temperature is raised to a minimum of 750.degree. C. but usually to not more than 1200.degree. C. peak temperature.
During the baking process, considerable quantities of volatile are evolved and considerable shrinkage of the material takes place. The binder material becomes pyrolized to form infusible carbon bonds among the filler particles. Carbon bodies baked to less than the graphitization temperature, e.g. about 1500.degree. C. are referred to as carbon-graphites. In some cases, the carbon bodies are further heat-treated in an inert atmosphere to as high as 3000.degree. C. for the purpose of purifying and/or graphitizing.
The carbon bodies produced in accordance with the above description are porous and have a high degree of permeability. In order to produce impervious components, the carbon bodies are vacuum-pressure impregnated with selected resins of known types. In some cases, the impregnated carbon bodies again are heat treated in an inert atmosphere to carbonize the resin. After heat treatment, the carbon bodies are again reimpregnated with resin to assure imperviousness.
Carbon material for mechanical seal applications are made by careful selection of filler and binder materials and following specific processing steps. Nevertheless, problems are often encountered during use, for example, in mechanical face seals, with carbon ring wearing and with grooving of the associated hard mating ring. Such problems arise especially at high operating temperatures and pressures, to the detriment of seal performance and durability.
Attempts to reduce seal face wear have included additive materials used in tribological applications to increase the ability of a sliding member to withstand increased wear at temperatures up to 1000.degree. C. Platon et al. in "Study of Tribological Behavior of Si-C and Si.sub.3 N.sub.4 Ceramic Couples in Terms of Temperature: The Reality" describe artificial bodies of solid lubricant type, such as cerium fluoride powder and plasma coatings. Additionally, solid lubricants are described by Sliney in an article entitled "Rare Earth Fluorides and Oxides: Their Uses as Solid Lubricants at Temperatures to 1800.degree. F." NASATECH NOTE, 1969, NASA-TN-D-5301 and also in U.S. Pat. No. 5,200,098.
Other solid lubricants, including cerium fluoride for use in the context of a high temperature low friction seal in automobiles are described in U.S. Pat. No. 4,951,954. Lubricants containing cerium trifluoride, lanthanum trifluoride, and neodymium trifluoride for use with a recording medium are described in U.S. Pat. No. 4,034,133. Rare earth fluoride lubricants for use in die casting components, such as cerium trifluoride and lanthanum trifluoride, are described in U.S. Pat. No. 3,830,280.